hmp: Simplify qom-set

Simplify qom_set by making it use qmp_qom_set and the JSON parser.

(qemu) qom-get /machine smm
"auto"
(qemu) qom-set /machine smm "auto"

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20200520151108.160598-3-dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  With 's'->'S' type change suggested by Paolo and Markus